spi: atmel-quadspi: Fix printed error code during DMA setup

spi: atmel-quadspi: Fix printed error code during DMA setup

On dma_request_chan() failure driver NULL-ifies the 'rx_chan' and
immediately uses it as PTR_ERR() so dev_err_probe() prints incorrect
error code.  Rework the code so proper error code will be printed and
NULL-ifying of 'rx_chan' will happen in common error handling block
(failure of DMA setup is not fatal for the driver and further code
depends on 'rx_chan' being non-NULL for DMA operations).

Reported by Smatch:
  drivers/spi/atmel-quadspi.c:1287 atmel_qspi_dma_init() warn: passing zero to 'PTR_ERR'

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://patch.msgid.link/20250501-n-smatch-fixes-v2-1-d2ad9c1f2e67@linaro.org
Signed-off-by: Mark Brown <broonie@kernel.org>